[wingnut60]

You actually need to weigh the truck and get a rear axle weight also. These figures will override the 1806--probably it will be less.
The 15600 is also probably the max wt of a towed vehicle, not including the truck. But, you have to consider the pin weight of a fifth wheel, and that is the problem. You could tow more trailer than you can fifth wheel...
Are the fiver weights you mention 'dry' weights or 'gross' weights? If dry, the problem is even worse for rear axle overloading.
You are also making a mistake in adding the 2 axles together, as the gross weight of you truck is around 10000lbs, not 15000. The real limit is the 2 rear tires at 7500lbs total. This is why you need a scale weight to find out WHAT YOUR TRUCK WILL WEIGH AS YOU INTEND TO USE IT UNDER THE FIVER--people, hitch, firewood, coolers, dogs, etc.
6100 rear axle = most your rear axle should/can weigh, not even the 7500 capacity of the tires. So, weigh the truck and see what is on the rear axle without a trailer, but loaded for a trip. Then, take that from 7500 the tire capacity--that is a much as I would go to traveling overloaded.
Whatever you decide, you will be overloaded on the rear axle, but using it as described might work out, if you travel slow and not very far.
After all the above--welcome to the 5thwheelforums.
Joe